STRUCTURE OF BLOOD-VESSELS. 



677 



^ Iiitima 



Media 



pulmonary artery, the media consists chiefly of striated muscle which resembles that 

 of the myocardium with which it is continuous, both vessels having been derived from 

 a common trunk, the bulbus arteriosus, the anterior segment of the primary heart-tube. 



The Veins. — The walls of the veins are always thinner than those of corre- 

 sponding arteries and are more flaccid and less contractile in consequence of the 

 smaller amount of elastic and muscular tissue that they contain. 



In veins of medium size (from 4-8 mm. in diameter), the intima consists of the 

 lining endothelium, the cells of which are relatively broad and short, a thin layer of 

 fibrous connective tissue and net-works of fine elastic fibres. A distinct internal 

 elastic membrane is seldom pres- 

 ent, at most a condensation of Fig. 638. 

 elastic fibrillae marking the outer 

 limit of the inner coat. In some 

 veins, as the cephalic, basilic, 

 femoral, long saphenous, and pop- 

 hteal, bundles of smooth muscle 

 occur within the intima. In ad- 

 dition to the circularly disposed 

 thin sheets of muscular and fibro- 

 elastic tissue, the media contains 

 fibro-elastic plates, sometimes 

 mingled with a few bundles of 

 muscle-cells, that extend longi- 

 tudinally. In certain veins, as in 

 the saphenous, deep femoral, and 

 popliteal, the longitudinal fibres 

 may constitute a zone beneath the 

 intima to the exclusion of the mus- 

 cular tissue. The adverititia is 

 often thicker than the media, and 

 consists of interlacing fibres and 

 net-works of fibro-elastic strands, 

 the general direction of which is 

 lengthwise. In many veins, par- 

 ticularly in those of the lower ex- 

 tremity, the outer coat contains 

 bundles of longitudinally disposed 

 muscle-cells. 



The valves with which many 

 veins are provided consist of 

 paired crescentic folds (Fig. 641) 

 of the intima, covered on both 

 sides with endothelium, containing 

 a small amount of fibro-elastic tis- 

 sue. The attached border of the 

 leaflets ends in narrow prolonga- 

 tions that extend beyond the free 

 margin of the valve. Between the 

 leaflets of the valve and the wall 

 of the vein lie the pocket-like si- 

 nuses, which the blood distends when the vah-e is closed. 



T Adventitia 



Areolar 

 tissue 



Transverse section of abdominal aorta. X 90. 



of their 

 While 



In the structure 

 walls, the large veins present many deviations from the typical arrangement, 

 the intima is only exceptionally increased, as in the hepatic part of the inferior vena 

 cava and the beginning of the portal vein, the media is often markedly thickened. 

 This increase is chiefly due to augmentation of the elastic and fibrous tissue, the mus- 

 cle remaining comparatively scanty. The splenic and portal veins, however, are 

 particularly rich in muscular tissue.; on the other hand, the media may be 

 almost wanting, as in the greater part of the inferior vena cava and the larger 

 hepatic veins. 



